FDC-Specific Functions of p55TNFR and IKK2 in the Development of FDC Networks and of Antibody Responses

Vagus nerve stimulation modulates distinct acetylcholine receptors on B cells and limits the germinal center response

Acetylcholine is produced in the spleen in response to vagus nerve activation; however, the effects on antibody production have been largely unexplored. Here, we use a chronic vagus nerve stimulation (VNS) mouse model to study the effect of VNS on T-dependent B cell responses. We observed lower titers of high-affinity IgG and fewer antigen-specific germinal center (GC) B cells. GC B cells from chronic VNS mice exhibited altered mRNA and protein expression suggesting increased apoptosis and impaired plasma cell differentiation. Follicular dendritic cell (FDC) cluster dispersal and altered gene expression suggested poor function. The absence of acetylcholine-producing CD4+ T cells diminished these alterations. In vitro studies revealed that α7 and α9 nicotinic acetylcholine receptors (nAChRs) directly regulated B cell production of TNF, a cytokine crucial to FDC clustering. α4 nAChR inhibited coligation of CD19 to the B cell receptor, presumably decreasing B cell survival. Thus, VNS-induced GC impairment can be attributed to distinct effects of nAChRs on B cells.

The G allele of SNP rs3922 reduces the binding affinity between IGF2BP3 and CXCR5 correlating with a lower antibody production

Effective vaccines that function through humoral immunity seek to produce high-affinity antibodies. Our previous research identified the single-nucleotide polymorphism rs3922G in the 3'UTR of CXCR5 as being associated with nonresponsiveness to the hepatitis B vaccine. The differential expression of CXCR5 between the dark zone (DZ) and light zone (LZ) is critical for organizing the functional structure of the germinal center (GC). In this study, we report that the RNA-binding protein IGF2BP3 can bind to CXCR5 mRNA containing the rs3922 variant to promote its degradation via the nonsense-mediated mRNA decay pathway. Deficiency of IGF2BP3 leads to increased CXCR5 expression, which results in the disappearance of CXCR5 differential expression between DZ and LZ, disorganized GCs, aberrant somatic hypermutations, and reduced production of high-affinity antibodies. Furthermore, the affinity of IGF2BP3 for the rs3922G-containing sequence is lower than that for the rs3922A counterpart, which may explain the nonresponsiveness to the hepatitis B vaccination. Together, our findings suggest that IGF2BP3 plays a crucial role in the production of high-affinity antibodies in the GC by binding to the rs3922-containing sequence to regulate CXCR5 expression.

Salmonella infection induces the reorganization of follicular dendritic cell networks concomitant with the failure to generate germinal centers

Germinal centers (GCs) are sites where plasma and memory B cells form to generate high-affinity, Ig class-switched antibodies. Specialized stromal cells called follicular dendritic cells (FDCs) are essential for GC formation. During systemic Salmonella Typhimurium (STm) infection GCs are absent, whereas extensive extrafollicular and switched antibody responses are maintained. The mechanisms that underpin the absence of GC formation are incompletely understood. Here, we demonstrate that STm induces a reversible disruption of niches within the splenic microenvironment, including the T and B cell compartments and the marginal zone. Alongside these effects after infection, mature FDC networks are strikingly absent, whereas immature FDC precursors, including marginal sinus pre-FDCs (MadCAM-1+) and perivascular pre-FDCs (PDGFRβ+) are enriched. As normal FDC networks re-establish, extensive GCs become detectable throughout the spleen. Therefore, the reorganization of FDC networks and the loss of GC responses are key, parallel features of systemic STm infections.

Foot-and-mouth disease virus localisation on follicular dendritic cells and sustained induction of neutralising antibodies is dependent on binding to complement receptors (CR2/CR1)

Previous studies have shown after the resolution of acute infection and viraemia, foot-and-mouth disease virus (FMDV) capsid proteins and/or genome are localised in the light zone of germinal centres of lymphoid tissue in cattle and African buffalo. The pattern of staining for FMDV proteins was consistent with the virus binding to follicular dendritic cells (FDCs). We have now demonstrated a similar pattern of FMDV protein staining in mouse spleens after acute infection and showed FMDV proteins are colocalised with FDCs. Blocking antigen binding to complement receptor type 2 and 1 (CR2/CR1) prior to infection with FMDV significantly reduced the detection of viral proteins on FDCs and FMDV genomic RNA in spleen samples. Blocking the receptors prior to infection also significantly reduced neutralising antibody titres, through significant reduction in their avidity to the FMDV capsid. Therefore, the binding of FMDV to FDCs and sustained induction of neutralising antibody responses are dependent on FMDV binding to CR2/CR1 in mice.

Context dependent induction of autoimmunity by TNF signaling deficiency

TNF inhibitors are widely used to treat inflammatory diseases; however, 30%–50% of treated patients develop new autoantibodies, and 0.5%–1% develop secondary autoimmune diseases, including lupus. TNF is required for formation of germinal centers (GCs), the site where high-affinity autoantibodies are often made. We found that TNF deficiency in Sle1 mice induced TH17 T cells and enhanced the production of germline encoded, T-dependent IgG anti-cardiolipin antibodies but did not induce GC formation or precipitate clinical disease. We then asked whether a second hit could restore GC formation or induce pathogenic autoimmunity in TNF-deficient mice. By using a range of immune stimuli, we found that somatically mutated autoantibodies and clinical disease can arise in the setting of TNF deficiency via extrafollicular pathways or via atypical GC-like pathways. This breach of tolerance may be due to defects in regulatory signals that modulate the negative selection of pathogenic autoreactive B cells.

Follicular dendritic cells

Follicular dendritic cells (FDCs) are unique accessory immune cells that contribute to the regulation of humoral immunity. They are multitasker cells essential for the organization and maintenance of the lymphoid architecture, induction of germinal center reaction, production of B memory cells, and protection from autoimmune disorders. They perform their activities through both antigen-driven and chemical signaling to B cells. FDCs play a crucial role in the physiological regulation of the immune response. Dis-regulation of this immune response results when FDCs retain antigens for years. This provides a constant antigenic stimulation for B cells resulting in the development of immune disorders. Antigen trapped on FDCs is resistant to therapeutic intervention causing chronicity and recurrences. Beyond their physiological immunoregulatory functions, FDCs are involved in the pathogenesis of several immune-related disorders including HIV/AIDS, prion diseases, chronic inflammatory, and autoimmune disorders. FDCs have also been recently implicated in rare neoplasms of lymphoid and hematopoietic tissues. Understanding FDC biology is essential for better control of humoral immunity and opens the gate for therapeutic management of FDC-mediated immune disorders. Thus, the biology of FDCs has become a hot research area in the last couple of decades. In this review, we aim to provide a comprehensive overview of FDCs and their role in physiological and pathological conditions.

Follicular dendritic cell dysfunction contributes to impaired antigen-specific humoral responses in sepsis-surviving mice

Sepsis survivors exhibit impaired responsiveness to antigen (Ag) challenge associated with increased mortality from infection. The contribution of follicular dendritic cells (FDCs) in the impaired humoral response in sepsis-surviving mice is investigated in this study. We demonstrated that mice subjected to sepsis from cecal ligation and puncture (CLP mice) have reduced NP-specific high-affinity class-switched Ig antibodies (Abs) compared with sham-operated control mice following immunization with the T cell-dependent Ag, NP-CGG. NP-specific germinal center (GC) B cells in CLP mice exhibited reduced TNF-α and AID mRNA expression compared with sham-operated mice. CLP mice showed a reduction in FDC clusters, a reduced binding of immune complexes on FDCs, and reduced mRNA expression of CR2, ICAM-1, VCAM-1, FcγRIIB, TNFR1, IKK2, and LTβR compared with sham-operated mice. Adoptive transfer studies showed that there was no B cell-intrinsic defect. In summary, our data suggest that the reduced Ag-specific Ab response in CLP mice is secondary to a disruption in FDC and GC B cell function.

Differentiation and activation of fibroblastic reticular cells

Secondary lymphoid organs (SLO) are underpinned by fibroblastic reticular cells (FRC) that form dedicated microenvironmental niches to secure induction and regulation of innate and adaptive immunity. Distinct FRC subsets are strategically positioned in SLOs to provide niche factors and govern efficient immune cell interaction. In recent years, the use of specialized mouse models in combination with single-cell transcriptomics has facilitated the elaboration of the molecular FRC landscape at an unprecedented resolution. While single-cell RNA-sequencing has advanced the resolution of FRC subset characterization and function, the high dimensionality of the generated data necessitates careful analysis and validation. Here, we reviewed novel findings from high-resolution transcriptomic analyses that refine our understanding of FRC differentiation and activation processes in the context of infection and inflammation. We further discuss concepts, strategies, and limitations for the analysis of single-cell transcriptome data from FRCs and the wide-ranging implications for our understanding of stromal cell biology.

Lymphoid stromal cells-more than just a highway to humoral immunity

The generation of high-affinity long-lived antibody responses is dependent on the differentiation of plasma cells and memory B cells, which are themselves the product of the germinal centre (GC) response. The GC forms in secondary lymphoid organs in response to antigenic stimulation and is dependent on the coordinated interactions between many types of leucocytes. These leucocytes are brought together on an interconnected network of specialized lymphoid stromal cells, which provide physical and chemical guidance to immune cells that are essential for the GC response. In this review we will highlight recent advancements in lymphoid stromal cell immunobiology and their role in regulating the GC, and discuss the contribution of lymphoid stromal cells to age-associated immunosenescence.

Fibroblastic reticular cell lineage convergence in Peyer's patches governs intestinal immunity

Fibroblastic reticular cells (FRCs) determine the organization of lymphoid organs and control immune cell interactions. While the cellular and molecular mechanisms underlying FRC differentiation in lymph nodes and the splenic white pulp have been elaborated to some extent, in Peyer's patches (PPs) they remain elusive. Using a combination of single-cell transcriptomics and cell fate mapping in advanced mouse models, we found that PP formation in the mouse embryo is initiated by an expansion of perivascular FRC precursors, followed by FRC differentiation from subepithelial progenitors. Single-cell transcriptomics and cell fate mapping confirmed the convergence of perivascular and subepithelial FRC lineages. Furthermore, lineage-specific loss- and gain-of-function approaches revealed that the two FRC lineages synergistically direct PP organization, maintain intestinal microbiome homeostasis and control anticoronavirus immune responses in the gut. Collectively, this study reveals a distinct mosaic patterning program that generates key stromal cell infrastructures for the control of intestinal immunity.

The Effects of Immune System Modulation on Prion Disease Susceptibility and Pathogenesis

Prion diseases are a unique group of infectious chronic neurodegenerative disorders to which there are no cures. Although prion infections do not stimulate adaptive immune responses in infected individuals, the actions of certain immune cell populations can have a significant impact on disease pathogenesis. After infection, the targeting of peripherally-acquired prions to specific immune cells in the secondary lymphoid organs (SLO), such as the lymph nodes and spleen, is essential for the efficient transmission of disease to the brain. Once the prions reach the brain, interactions with other immune cell populations can provide either host protection or accelerate the neurodegeneration. In this review, we provide a detailed account of how factors such as inflammation, ageing and pathogen co-infection can affect prion disease pathogenesis and susceptibility. For example, we discuss how changes to the abundance, function and activation status of specific immune cell populations can affect the transmission of prion diseases by peripheral routes. We also describe how the effects of systemic inflammation on certain glial cell subsets in the brains of infected individuals can accelerate the neurodegeneration. A detailed understanding of the factors that affect prion disease transmission and pathogenesis is essential for the development of novel intervention strategies.

Influence of the Draining Lymph Nodes and Organized Lymphoid Tissue Microarchitecture on Susceptibility to Intradermal Trypanosoma brucei Infection

Infection of the mammalian host with African trypanosomes begins when the tsetse fly vector injects the parasites into the skin dermis during blood feeding. After injection into the skin, trypanosomes first accumulate in the draining lymph node before disseminating systemically. Whether this early accumulation within the draining lymph node is important for the trypanosomes to establish infection was not known. Lymphotoxin-β-deficient mice (LTβ^-/- mice) lack most secondary lymphoid tissues, but retain the spleen and mesenteric lymph nodes. These mice were used to test the hypothesis that the establishment of infection after intradermal (ID) T. brucei infection would be impeded in the absence of the skin draining lymph nodes. However, LTβ^-/- mice revealed greater susceptibility to ID T. brucei infection than wild-type mice, indicating that the early accumulation of the trypanosomes in the draining lymph nodes was not essential to establish systemic infection. Although LTβ^-/- mice were able to control the first parasitemia wave as effectively as wild-type mice, they were unable to control subsequent parasitemia waves. LTβ^-/- mice also lack organized B cell follicles and germinal centers within their remaining secondary lymphoid tissues. As a consequence, LTβ^-/- mice have impaired immunoglobulin (Ig) isotype class-switching responses. When the disturbed microarchitecture of the B cell follicles in the spleens of LTβ^-/- mice was restored by reconstitution with wild-type bone marrow, their susceptibility to ID T. brucei infection was similar to that of wild-type control mice. This effect coincided with the ability to produce significant serum levels of Ig isotype class-switched parasite-specific antibodies. Thus, our data suggest that organized splenic microarchitecture and the production of parasite-specific Ig isotype class-switched antibodies are essential for the control of ID African trypanosome infections.

Stromal cell control of conventional and ectopic germinal centre reactions

The germinal centre (GC) is a specialized cellular structure that forms in response to antigenic stimulation. It generates long-term humoral immunity through the production of memory B cells and long-lived antibody-secreting plasma cells. Conventional GCs form within secondary lymphoid organs, where networks of specialised stromal cells that form during embryogenesis act as the stage upon which the various GC immune cell players are brought together, nurtured and co-ordinated to generate a productive response. In non-lymphoid organs, ectopic GCs can form in response to persistent antigenic and inflammatory stimuli. Unlike secondary lymphoid tissues, non-lymphoid organs do not have a developmentally programmed stromal cell network capable of supporting the germinal centre reaction; therefore, the local tissue stroma must be remodelled by inflammatory stimuli in order to host a GC reaction. These ectopic GCs produce memory B cells and plasma cells that form a critical component of the humoral immune response.

Distinct modes of TNF signaling through its two receptors in health and disease

TNF is a key proinflammatory and immunoregulatory cytokine whose deregulation is associated with the development of autoimmune diseases and other pathologies. Recent studies suggest that distinct functions of TNF may be associated with differential engagement of its two receptors: TNFR1 or TNFR2. In this review, we discuss the relative contributions of these receptors to pathogenesis of several diseases, with the focus on autoimmunity and neuroinflammation. In particular, we discuss the role of TNFRs in the development of regulatory T cells during neuroinflammation and recent findings concerning targeting TNFR2 with agonistic and antagonistic reagents in various murine models of autoimmune and neuroinflammatory disorders and cancer.

Safe eradication of large established tumors using neovasculature-targeted tumor necrosis factor-based therapies

Systemic toxicities have severely limited the clinical application of tumor necrosis factor (TNF) as an anticancer agent. Activity‐on‐Target cytokines (AcTakines) are a novel class of immunocytokines with improved therapeutic index. A TNF‐based AcTakine targeted to CD13 enables selective activation of the tumor neovasculature without any detectable toxicity in vivo. Upregulation of adhesion markers supports enhanced T‐cell infiltration leading to control or elimination of solid tumors by, respectively, CAR T cells or a combination therapy with CD8‐targeted type I interferon AcTakine. Co‐treatment with a CD13‐targeted type II interferon AcTakine leads to very rapid destruction of the tumor neovasculature and complete regression of large, established tumors. As no tumor markers are needed, safe and efficacious elimination of a broad range of tumor types becomes feasible.

Germinal center B cell initiation, GC maturation, and the coevolution of its stromal cell niches

Throughout the developing GC response, B cell survival and fate choices made at the single cell level are dependent on signals received largely through interactions with other cells, often with cognate T cells. The type of signals that a given B cell can encounter is dictated by its location within tissue microarchitecture. The focus of this review is on the initiation and evolution of the GC response at the earliest time points. Here, we review the key factors influencing the progression of GC B cell differentiation that are both stage and context dependent. Finally, we describe the coevolution of niches within and surrounding the GC that influence the outcome of the GC response.

B Cell Receptor Crosslinking Augments Germinal Center B Cell Selection when T Cell Help Is Limiting

Antigen-dependent engagement of germinal center (GC) B cell receptors (BCRs) promotes antigen internalization and presentation for follicular helper T cells. However, whether BCR signaling is critical or synergistic with T cell help for GC B cell selection or differentiation is unclear. Here, by adapting an experimental approach that enables independent delivery of BCR-crosslinking antigen or T cell help to GC B cells in vivo, we showed that T cell help was sufficient to induce GC B cell expansion and plasmablast formation. However, although BCR crosslinking could not by itself promote GC B cell selection or differentiation, it could synergize with T cell help to enhance the GC and plasmablast responses when T cell help was limiting. These findings indicate that GC B cells can integrate variable inputs from T cell help and BCR signaling in vivo for an optimal process of selection and differentiation, critical for potent long-term humoral immunity.

Essential Role of Canonical NF-κB Activity in the Development of Stromal Cell Subsets in Secondary Lymphoid Organs

Organized tissue structure in the secondary lymphoid organs (SLOs) tightly depends on the development of fibroblastic stromal cells (FSCs) of mesenchymal origin; however, the mechanisms of this relationship are poorly understood. In this study, we specifically inactivated the canonical NF-κB pathway in FSCs in vivo by conditionally inducing IκBα mutant in a Ccl19-IκBSR mouse system in which NF-κB activity is likely to be suppressed in fetal FSC progenitors. Given that NF-κB activation in fetal FSCs is essential for SLO development, the animals were expected to lack SLOs. However, all SLOs were preserved in Ccl19-IκBSR mice. Instead, the T cell area was severely disturbed by the lack of CCL21-expressing FSCs, whereas the follicles and associated FSC networks were formed. Fate mapping revealed that IκBSR-expressing cells constituted only a small fraction of stromal compartment outside the follicles. Taken together, our findings indicate an essential role of the canonical NF-κB pathway activity in the development of three FSC subsets common to SLOs and suggest transient or stochastic CCL19 expression in FSC progenitors and a compensatory differentiation program of follicular FSCs.

Regulation of the Germinal Center Response

The germinal center (GC) is a specialized microstructure that forms in secondary lymphoid tissues, producing long-lived antibody secreting plasma cells and memory B cells, which can provide protection against reinfection. Within the GC, B cells undergo somatic mutation of the genes encoding their B cell receptors which, following successful selection, can lead to the emergence of B cell clones that bind antigen with high affinity. However, this mutation process can also be dangerous, as it can create autoreactive clones that can cause autoimmunity. Because of this, regulation of GC reactions is critical to ensure high affinity antibody production and to enforce self-tolerance by avoiding emergence of autoreactive B cell clones. A productive GC response requires the collaboration of multiple cell types. The stromal cell network orchestrates GC cell dynamics by controlling antigen delivery and cell trafficking. T follicular helper (Tfh) cells provide specialized help to GC B cells through cognate T-B cell interactions while Foxp3⁺ T follicular regulatory (Tfr) cells are key mediators of GC regulation. However, regulation of GC responses is not a simple outcome of Tfh/Tfr balance, but also involves the contribution of other cell types to modulate the GC microenvironment and to avoid autoimmunity. Thus, the regulation of the GC is complex, and occurs at multiple levels. In this review we outline recent developments in the biology of cell subsets involved in the regulation of GC reactions, in both secondary lymphoid tissues, and Peyer's patches (PPs). We discuss the mechanisms which enable the generation of potent protective humoral immunity whilst GC-derived autoimmunity is avoided.

Synaptic Interactions in Germinal Centers

The germinal center (GC) is a complex, highly dynamic microanatomical niche that allows the generation of high-affinity antibody-producing plasma cells and memory B cells. These cells constitute the basis of long-lived highly protective antibody responses. For affinity maturation to occur, B cells undergo multiple rounds of proliferation and mutation of the genes that encode the immunoglobulin V region followed by selection by specialized T cells called follicular helper T (T_FH) cells. In order to achieve this result, the GC requires spatially and temporally coordinated interactions between the different cell types, including B and T lymphocytes and follicular dendritic cells. Cognate interactions between T_FH and GC B cells resemble cellular connections and synaptic communication within the nervous system, which allow signals to be transduced rapidly and effectively across the synaptic cleft. Such immunological synapses are particularly critical in the GC where the speed of T–B cell interactions is faster and their duration shorter than at other sites. In addition, the antigen-based specificity of cognate interactions in GCs is critical for affinity-based selection in which B cells compete for T cell help so that rapid modulation of the signaling threshold determines the outcome of the interaction. In the context of GCs, which contain large numbers of cells in a highly compacted structure, focused delivery of signals across the interacting cells becomes particularly important. Promiscuous or bystander delivery of positive selection signals could potentially lead to the appearance of long-lived self-reactive B cell clones. Cytokines, cytotoxic granules, and more recently neurotransmitters have been shown to be transferred from T_FH to B cells upon cognate interactions. This review describes the current knowledge on immunological synapses occurring during GC responses including the type of granules, their content, and function in T_FH-mediated help to B cells.

Tumour necrosis factor receptor-1 is dispensable for the migration of marginal metallophilic macrophages into the B-cell zone of the mouse spleen

The spleen is the only blood filter in the organism which removes foreign antigens and effete cells from circulation. The significant role in capturing, transporting and presentation of antigens to immune cells is executed by a special subset of splenic macrophages called marginal metallophilic macrophages. Upon stimulation with lipopolysaccharide, these cells promptly migrate from their preferential location at the inner aspect of the splenic marginal sinus into the B-cell lymphoid follicles. This migration is executed via CXC chemokine ligand 13 in a lymphotoxin-dependent fashion. However, the role of tumour necrosis factor-α/tumour necrosis factor receptor-1 signalling axis has not been studied, despite its critical role in the formation of B-cell lymphoid follicles, follicular dendritic cell networks and germinal centres. Here, we show that signalling via tumour necrosis factor receptor-1 is not required for the migration of marginal metallophilic macrophages into the B-cell zone and that the presence of organized B-cell lymphoid follicles is not a prerequisite for their dislocation.

Notch signaling represents an important checkpoint between follicular T-helper and canonical T-helper 2 cell fate

Type-2 immunity is regulated by two distinct CD4+ T-cell subsets. T follicular helper (Tfh) cells are required for humoral hallmarks of type-2 inflammation. T-helper type-2 (Th2) cells orchestrate type-2 inflammation in peripheral tissues, such as the lung and intestine. Given the importance of Notch signaling in the establishment of other CD4+ T-helper cell subsets, we investigated whether canonical Notch activation could differentially impact Tfh and Th2 cell fate during the induction of type-2 immunity. These studies show that Tfh cell, but not Th2 cell, generation and function is reliant on Notch signaling. While early Tfh cell specification is influenced by functional Notch ligands on classical dendritic cells, functional Notch ligands on cells other than dendritic cells, T cells, B cells, and follicular dendritic cells are sufficient to achieve full Tfh cell commitment. These findings identify Notch signaling as an early lineage-determining factor between Tfh and Th2 cell fate.

Mesenchymal TNFR2 promotes the development of polyarthritis and comorbid heart valve stenosis

Mesenchymal TNF signaling is etiopathogenic for inflammatory diseases such as rheumatoid arthritis and spondyloarthritis (SpA). The role of Tnfr1 in arthritis has been documented; however, Tnfr2 functions are unknown. Here, we investigate the mesenchymal-specific role of Tnfr2 in the TnfΔARE mouse model of SpA in arthritis and heart valve stenosis comorbidity by cell-specific, Col6a1-cre-driven gene targeting. We find that TNF/Tnfr2 signaling in resident synovial fibroblasts (SFs) and valvular interstitial cells (VICs) is detrimental for both pathologies, pointing to common cellular mechanisms. In contrast, systemic Tnfr2 provides protective signaling, since its complete deletion leads to severe deterioration of both pathologies. SFs and VICs lacking Tnfr2 fail to acquire pathogenic activated phenotypes and display increased expression of antiinflammatory cytokines associated with decreased Akt signaling. Comparative RNA sequencing experiments showed that the majority of the deregulated pathways in TnfΔARE mesenchymal-origin SFs and VICs, including proliferation, inflammation, migration, and disease-specific genes, are regulated by Tnfr2; thus, in its absence, they are maintained in a quiescent nonpathogenic state. Our data indicate a pleiotropy of Tnfr2 functions, with mesenchymal Tnfr2 driving cell activation and arthritis/valve stenosis pathogenesis only in the presence of systemic Tnfr2, whereas nonmesenchymal Tnfr2 overcomes this function, providing protective signals and, thus, containing both pathologies.

Comorbid TNF-mediated heart valve disease and chronic polyarthritis share common mesenchymal cell-mediated aetiopathogenesis

Objectives

Patients with rheumatoid arthritis and spondyloarthritisshow higher mortality rates, mainly caused by cardiac comorbidities. The TghuTNF (Tg197) arthritis model develops tumour necrosis factor (TNF)-driven and mesenchymalsynovial fibroblast (SF)-dependent polyarthritis. Here, we investigate whether this model develops, similarly to human patients, comorbid heart pathology and explore cellular and molecular mechanisms linking arthritis to cardiac comorbidities.

Methods

Histopathological analysis and echocardiographic evaluation of cardiac function were performed in the Tg197 model. Valve interstitial cells (VICs) were targeted by mice carrying the ColVI-Cretransgene. Tg197 ColVI-Cre Tnfr1^fl/fl and Tg197 ColVI-Cre Tnfr1^cneo/cneo mutant mice were used to explore the role of mesenchymal TNF signalling in the development of heart valve disease. Pathogenic VICs and SFs were further analysed by comparative RNA-sequencing analysis.

Results

Tg197 mice develop left-sided heart valve disease, characterised by valvular fibrosis with minimal signs of inflammation. Thickened valve areas consist almost entirely of hyperproliferative ColVI-expressing mesenchymal VICs. Development of pathology results in valve stenosis and left ventricular dysfunction, accompanied by arrhythmic episodes and, occasionally, valvular regurgitation. TNF dependency of the pathology was indicated by disease modulation following pharmacological inhibition or mesenchymal-specific genetic ablation or activation of TNF/TNFR1 signalling. Tg197-derived VICs exhibited an activated phenotype ex vivo, reminiscent of the activated pathogenic phenotype of Tg197-derived SFs. Significant functional similarities between SFs and VICs were revealed by RNA-seq analysis, demonstrating common cellular mechanisms underlying TNF-mediated arthritides and cardiac comorbidities.

Conclusions

Comorbidheart valve disease and chronic polyarthritis are efficiently modelled in the Tg197 arthritis model and share common TNF/TNFR1-mediated, mesenchymal cell-specific aetiopathogenic mechanisms.

How do PrPSc Prions Spread between Host Species, and within Hosts?

Prion diseases are sub-acute neurodegenerative diseases that affect humans and some domestic and free-ranging animals. Infectious prion agents are considered to comprise solely of abnormally folded isoforms of the cellular prion protein known as PrP^Sc. Pathology during prion disease is restricted to the central nervous system where it causes extensive neurodegeneration and ultimately leads to the death of the host. The first half of this review provides a thorough account of our understanding of the various ways in which PrP^Sc prions may spread between individuals within a population, both horizontally and vertically. Many natural prion diseases are acquired peripherally, such as by oral exposure, lesions to skin or mucous membranes, and possibly also via the nasal cavity. Following peripheral exposure, some prions accumulate to high levels within the secondary lymphoid organs as they make their journey from the site of infection to the brain, a process termed neuroinvasion. The replication of PrP^Sc prions within secondary lymphoid organs is important for their efficient spread to the brain. The second half of this review describes the key tissues, cells and molecules which are involved in the propagation of PrP^Sc prions from peripheral sites of exposure (such as the lumen of the intestine) to the brain. This section also considers how additional factors such as inflammation and aging might influence prion disease susceptibility.

Follicular Dendritic Cell Activation by TLR Ligands Promotes Autoreactive B Cell Responses

A hallmark of autoimmunity in murine models of lupus is the formation of germinal centers (GCs) in lymphoid tissues where self-reactive B cells expand and differentiate. In the host response to foreign antigens, follicular dendritic cells (FDCs) maintain GCs through the uptake and cycling of complement-opsonized immune complexes. Here, we examined whether FDCs retain self-antigens and the impact of this process in autoantibody secretion in lupus. We found that FDCs took up and retained self-immune complexes composed of ribonucleotide proteins, autoantibody, and complement. This uptake, mediated through CD21, triggered endosomal TLR7 and led to the secretion of interferon (IFN) α via an IRF5-dependent pathway. Blocking of FDC secretion of IFN-α restored B cell tolerance and reduced the amount of GCs and pathogenic autoantibody. Thus, FDCs are a critical source of the IFN-α driving autoimmunity in this lupus model. This pathway is conserved in humans, suggesting that it may be a viable therapeutic target in systemic lupus erythematosus.

Stromal networking: cellular connections in the germinal centre

Secondary lymphoid organs are organized into distinct zones, governed by different types of mesenchymal stromal cells. These stromal cell subsets are critical for the generation of protective humoral immunity because they direct the migration of, and interaction between, multiple immune cell types to form the germinal centre. The germinal centre response generates long-lived antibody-secreting plasma cells and memory B cells which can provide long-term protection against re-infection. Stromal cell subsets mediate this response through control of immune cell trafficking, activation, localization and antigen access within the secondary lymphoid organ. Further, distinct populations of stromal cells underpin the delicate spatial organization of immune cells within the germinal centre. Because of this, the interactions between immune cells and stromal cells in secondary lymphoid organs are fundamental to the germinal centre response. Herein we review how this unique relationship leads to effective germinal centre responses.

Modular Immune Organoids with Integrin Ligand Specificity Differentially Regulate Ex Vivo B Cell Activation

Germinal centers are dynamic structures within lymphoid tissues, which develop once B cells receive activating signals from surrounding immune cells. Germinal center B cells are small in number, heterogeneous, and prone to rapid apoptosis unless selected by the body to form memory B cells. Despite extensive research in the B cell differentiation process, the role of the lymphoid niche, in particular integrin ligands, in the development of early germinal center-like phenotype remains unclear. Here, we report a biomaterials-based modular immune organoid that enables development of early germinal-center phenotype in an integrin ligand-specific manner. We demonstrate the differential role of integrin α4β1- and αvβ3-binding ligands in the induction of GL7+ (GC-like) and GL7- (non-GC-like) phenotype in differentiating B cells while in the presence of CD40 ligand and interleukin-4. We further demonstrate the role of integrin ligand specificities in clustering of β3 integrin and B cell receptor on the surface of differentiated B cells in 3D organoids as compared to the classic 2D cocultures. The study demonstrates that biomaterials-based immune organoids represent an ex vivo platform technology, which recapitulates certain aspects of GC biology to understand the process of B cell differentiation and induction of immunological responses. This platform is particularly useful in understanding the role of selective biomolecular signals and the temporal dependency of immune responses to these signals.

Immuno-engineered organoids for regulating the kinetics of B-cell development and antibody production

Induction of B-cell immunity against infection depends on the initiation of the germinal center (GC) reaction in secondary lymphoid organs. Ex vivo recapitulation of the GC reaction in 2D cultures results in transient cell growth, with poor yield and short-term survival. Furthermore, no reported 2D ex vivo system can modulate the kinetics of a GC-like phenotype or the rate of antibody class switching. This protocol describes a methodology for developing immune organoids that partially mimic the B-cell zone of a lymphoid tissue, for efficient and rapid generation of B cells with a GC-like phenotype from naive murine B cells. The organoid is composed of a bioadhesive protein, gelatin, that is transformed into an ionically cross-linked hydrated network using biocompatible silicate nanoparticles (SiNPs). We explain how to establish the immune organoid culture to sustain immune cell proliferation and transformation into a GC-like phenotype. Starting with cell encapsulation in digested lymphoid tissues, clusters of proliferating B cells with a GC-like phenotype can be generated in the organoids at controlled rates, within ∼1 week. The culture methodology described here is currently the only one that allows the accelerated induction of a GC-like phenotype in B cells and supports a controllable immunoglobulin class-switching reaction. This method can be easily implemented in a typical tissue culture room by personnel with standard mammalian cell culture expertise.

c-Rel is dispensable for the differentiation and functional maturation of M cells in the follicle-associated epithelium

M cells reside within the follicle-associated epithelium (FAE) overlying the gut-associated lymphoid tissues. These unique phagocytic epithelial cells enable the mucosal immune system to sample antigens within the lumen of the intestine. The differentiation of M cells from uncommitted precursors in the FAE is dependent on the production of receptor activator of nuclear factor-κB ligand (RANKL) by subepithelial stromal cells. The ligation of a variety of cell surface receptors activates the nuclear factor-κB (NF-κB) family of transcription factors which in-turn induce the transcription of multiple target genes. RANKL-stimulation can stimulate the nuclear translocation of the NF-κB subunit c-Rel. We therefore used c-Rel-deficient mice to determine whether the differentiation and functional maturation of M cells in the Peyer's patches was dependent on c-Rel. Our data show that c-Rel-deficiency does not influence the expression of RANKL or RANK in Peyer's patches, or the induction of M-cell differentiation in the FAE. RANKL-stimulation in the differentiating M cells induces the expression of SpiB which is essential for their subsequent maturation. However, SpiB expression in the FAE was also unaffected in the absence of c-Rel. As a consequence, the functional maturation of M cells was not impaired in the Peyer's patches of c-Rel-deficient mice. Although our data showed that the specific expression of CCL20 and ubiquitin D in the FAE was not impeded in the absence of c-Rel, the expression of ubiquitin D was dramatically reduced in the B cell-follicles of c-Rel-deficient mice. Coincident with this, we also observed that the status of follicular dendritic cells in the B cell-follicles was dramatically reduced in Peyer's patches from c-Rel-deficient mice. Taken together, our data show that c-Rel is dispensable for the RANKL-mediated differentiation and functional maturation of M cells.

CollagenVI-Cre mice: A new tool to target stromal cells in secondary lymphoid organs

Stromal cells in secondary lymphoid organs (SLOs) are non-hematopoietic cells involved in the regulation of adaptive immune responses. Three major stromal populations have been identified in adult SLOs: fibroblastic reticular cells (FRCs), follicular dendritic cells (FDCs) and marginal reticular cells (MRCs). The properties of these individual populations are not clearly defined, mainly due to the lack of appropriate genetic tools, especially for MRCs. Here, we analyzed stromal cell targeting in SLOs from a transgenic mouse strain that expresses Cre recombinase under the CollagenVI promoter, using lineage tracing approaches. We show that these mice target specifically MRCs and FDCs, but not FRCs in Peyer’s patches and isolated lymphoid follicles in the intestine. In contrast, stromal cells in lymph nodes and the spleen do not express the transgene, which renders ColVI-cre mice ideal for the specific targeting of stromal cells in the gut-associated lymphoid tissue (GALT). This funding further supports the hypothesis of organ-specific stromal precursors in SLOs. Interestingly, in all tissues analyzed, there was also high specificity for perivascular cells, which have been proposed to act as FDC precursors. Taken together, ColVI-Cre mice are a useful new tool for the dissection of MRC- and FDC-specific functions and plasticity in the GALT.

Neuroinflammatory TNFα Impairs Memory via Astrocyte Signaling

The occurrence of cognitive disturbances upon CNS inflammation or infection has been correlated with increased levels of the cytokine tumor necrosis factor-α (TNFα). To date, however, no specific mechanism via which this cytokine could alter cognitive circuits has been demonstrated. Here, we show that local increase of TNFα in the hippocampal dentate gyrus activates astrocyte TNF receptor type 1 (TNFR1), which in turn triggers an astrocyte-neuron signaling cascade that results in persistent functional modification of hippocampal excitatory synapses. Astrocytic TNFR1 signaling is necessary for the hippocampal synaptic alteration and contextual learning-memory impairment observed in experimental autoimmune encephalitis (EAE), an animal model of multiple sclerosis (MS). This process may contribute to the pathogenesis of cognitive disturbances in MS, as well as in other CNS conditions accompanied by inflammatory states or infections.

TNFR1-induced lethal inflammation is mediated by goblet and Paneth cell dysfunction

Tumor necrosis factor (TNF) is a powerful activator of the immune system and a well-validated target for treatment of autoimmune diseases. Injection of TNF induces systemic lethal inflammation characterized by hypothermia, induction of multiple cytokines, and extensive damage to multiple organs. Previously, we reported that TNF-induced lethal inflammation is strictly TNFR1(P55)-dependent. We also uncovered a crucial role for P55 expression levels in intestinal epithelial cells (IECs), in which P55+/+ expression is sufficient to sensitize to TNF lethality in an otherwise fully protected P55+/- background. Here, we investigated the molecular mechanism that drives TNF toxicity in IECs. Unexpectedly, we found that the degree of TNF-induced enterocyte damage and apoptosis in IECs is equally strong in TNF-sensitive P55+/+ mice and TNF-resistant P55+/- mice. Our results suggest that P55+/+-induced signaling causes goblet and Paneth cell dysfunction, leading to severe epithelial barrier dysfunction. As a result, intestinal permeability and systemic bacterial spread are induced, causing lethal systemic inflammation. In conclusion, we identified P55-induced goblet and Paneth cell dysfunction as a crucial mechanism for TNF-induced systemic and lethal inflammation.

Dysbiosis-induced intestinal inflammation activates tumor necrosis factor receptor I and mediates alcoholic liver disease in mice

Intestinal barrier dysfunction is an important contributor to alcoholic liver disease (ALD). Translocated microbial products trigger an inflammatory response in the liver and contribute to steatohepatitis. Our aim was to investigate mechanisms of barrier disruption after chronic alcohol feeding. A Lieber-DeCarli model was used to induce intestinal dysbiosis, increased intestinal permeability, and liver disease in mice. Alcohol feeding for 8 weeks induced intestinal inflammation in the jejunum, which is characterized by an increased number of tumor necrosis factor alpha (TNF-α)-producing monocytes and macrophages. These findings were confirmed in duodenal biopsies from patients with chronic alcohol abuse. Intestinal decontamination with nonabsorbable antibiotics restored eubiosis, decreased intestinal inflammation and permeability, and reduced ALD in mice. TNF-receptor I (TNFRI) mutant mice were protected from intestinal barrier dysfunction and ALD. To investigate whether TNFRI on intestinal epithelial cells mediates intestinal barrier dysfunction and ALD, we used TNFRI mutant mice carrying a conditional gain-of-function allele for this receptor. Reactivation of TNFRI on intestinal epithelial cells resulted in increased intestinal permeability and liver disease that is similar to wild-type mice after alcohol feeding, suggesting that enteric TNFRI promotes intestinal barrier dysfunction. Myosin light-chain kinase (MLCK) is a downstream target of TNF-α and was phosphorylated in intestinal epithelial cells after alcohol administration. Using MLCK-deficient mice, we further demonstrate a partial contribution of MLCK to intestinal barrier dysfunction and liver disease after chronic alcohol feeding.

CONCLUSION

Dysbiosis-induced intestinal inflammation and TNFRI signaling in intestinal epithelial cells mediate a disruption of the intestinal barrier. Therefore, intestinal TNFRI is a crucial mediator of ALD.

MicroRNA-100-5p indirectly modulates the expression of Il6, Ptgs1/2 and Tlr4 mRNA in the mouse follicular dendritic cell-like cell line, FL-Y

Follicular dendritic cells (FDC) are important stromal cells within the B-cell follicles and germinal centres (GC) of secondary lymphoid tissues. FDC trap and retain native antigens on their surfaces in the form of immune complexes that they display to B cells, in order to select those cells with the highest antigen affinity. MicroRNAs are short, non-coding RNAs that are approximately 18-25 nucleotides in length that regulate gene expression at the post-transcriptional level by repressing the translation of target genes. In the current study, in vivo and in vitro systems were used to identify microRNAs that were potentially expressed by FDC. Constitutive lymphotoxin-β receptor (LTβR) stimulation is required to maintain FDC in their differentiated state. We show that the rapid de-differentiation of spleen FDC that accompanied LTβR-blockade, coincided with a significant decrease in the expression of mmu-miR-100-5p, mmu-miR-138-5p and mmu-miR-2137. These microRNAs were shown to be expressed in the FDC-like cell line, FL-YB, and specific inhibition of mmu-miR-100-5p significantly enhanced expression of Il6, Ptgs1/2 and Tlr4 mRNA in this cell line. The expression of Il6, Ptgs1/2 and Tlr4 by FDC play important roles in regulating GC size and promoting high-affinity antibody responses, so it is plausible that mmu-miR-100-5p may help to regulate the expression of these genes during GC reactions.

TNFα-dependent development of lymphoid tissue in the absence of RORγt⁺ lymphoid tissue inducer cells

Lymphoid tissue often forms within sites of chronic inflammation. Here we report that expression of the proinflammatory cytokine tumor necrosis factor α (TNFα) drives development of lymphoid tissue in the intestine. Formation of this ectopic lymphoid tissue was not dependent on the presence of canonical RORgt(+) lymphoid tissue-inducer (LTi) cells, because animals expressing increased levels of TNFα but lacking RORgt(+) LTi cells (TNF/Rorc(gt)(-/-) mice) developed lymphoid tissue in inflamed areas. Unexpectedly, such animals developed several lymph nodes (LNs) that were structurally and functionally similar to those of wild-type animals. TNFα production by F4/80(+) myeloid cells present within the anlagen was important for the activation of stromal cells during the late stages of embryogenesis and for the activation of an organogenic program that allowed the development of LNs. Our results show that lymphoid tissue organogenesis can occur in the absence of LTi cells and suggest that interactions between TNFα-expressing myeloid cells and stromal cells have an important role in secondary lymphoid organ formation.

Integrin-mediated interactions between B cells and follicular dendritic cells influence germinal center B cell fitness

Integrin-ligand interactions between germinal center (GC) B cells and Ag-presenting follicular dendritic cells (FDCs) have been suggested to play central roles during GC responses, but their in vivo requirement has not been directly tested. In this study, we show that, whereas integrins αLβ2 and α4β1 are highly expressed and functional on mouse GC B cells, removal of single integrins or their ligands had little effect on B cell participation in the GC response. Combined β2 integrin deficiency and α4 integrin blockade also did not affect the GC response against a particulate Ag. However, the combined integrin deficiency did cause B cells to be outcompeted in splenic GC responses against a soluble protein Ag and in mesenteric lymph node GC responses against gut-derived Ags. Similar findings were made for β2-deficient B cells in mice lacking VCAM1 on FDCs. The reduced fitness of the GC B cells did not appear to be due to decreased Ag acquisition, proliferation rates, or pAKT levels. In summary, our findings provide evidence that αLβ2 and α4β1 play overlapping and context-dependent roles in supporting interactions with FDCs that can augment the fitness of responding GC B cells. We also find that mouse GC B cells upregulate αvβ3 and adhere to vitronectin and milk-fat globule epidermal growth factor VIII protein. Integrin β3-deficient B cells contributed in a slightly exaggerated manner to GC responses, suggesting this integrin has a regulatory function in GC B cells.

Follicular dendritic cells: origin, phenotype, and function in health and disease

Follicular dendritic cells (FDCs) were originally identified by their specific morphology and by their ability to trap immune-complexed antigen in B cell follicles. By virtue of the latter as well as the provision of chemokines, adhesion molecules, and trophic factors, FDCs participate in the shaping of B cell responses. Importantly, FDCs also supply tingible body macrophages (TBMs) with the eat-me-signaling molecule milk fat globule-EGF factor 8 (Mfge8), thereby enabling the disposal of apoptotic B cells. Recent studies have provided fundamental insights into the multiple functions of FDCs in both physiological and pathophysiological contexts and into their origin. Here we review these findings, and discuss current concepts related to FDC histogenesis both in lymphoid organs and in inflammatory lymphoneogenesis.

Tumor necrosis factor receptor signaling in keratinocytes triggers interleukin-24-dependent psoriasis-like skin inflammation in mice

Psoriasis is a common chronic inflammatory skin disease with a prevalence of about 2% in the Caucasian population. Tumor necrosis factor (TNF) plays an essential role in the pathogenesis of psoriasis, but its mechanism of action remains poorly understood. Here we report that the development of psoriasis-like skin inflammation in mice with epidermis-specific inhibition of the transcription factor NF-κB was triggered by TNF receptor 1 (TNFR1)-dependent upregulation of interleukin-24 (IL-24) and activation of signal transducer and activator of transcription 3 (STAT3) signaling in keratinocytes. IL-24 was strongly expressed in human psoriatic epidermis, and pharmacological inhibition of NF-κB increased IL-24 expression in TNF-stimulated human primary keratinocytes, suggesting that this mechanism is relevant for human psoriasis. Therefore, our results expand current views on psoriasis pathogenesis by revealing a new keratinocyte-intrinsic mechanism that links TNFR1, NF-κB, ERK, IL-24, IL-22R1, and STAT3 signaling to disease initiation.

Identification of a new stromal cell type involved in the regulation of inflamed B cell follicles

Identification of a new stromal cell type in mouse lymph nodes that can be activated by B cells to delineate the transient boundaries of B cell zones during inflammation

Lymph node (LN) stromal cells provide survival signals and adhesive substrata to lymphocytes. During an immune response, B cell follicles enlarge, questioning how LN stromal cells manage these cellular demands. Herein, we used a murine fate mapping system to describe a new stromal cell type that resides in the T cell zone of resting LNs. We demonstrated that upon inflammation, B cell follicles progressively trespassed into the adjacent T cell zone and surrounded and converted these stromal cells into CXCL13 secreting cells that in return delineated the new boundaries of the growing follicle. Acute B cell ablation in inflamed LNs abolished CXCL13 secretion in these cells, while LT-β deficiency in B cells drastically affected this conversion. Altogether, we reveal the existence of a dormant stromal cell subset that can be functionally awakened by B cells to delineate the transient boundaries of their expanding territories upon inflammation.

Immune responses develop in lymphoid organs such as the tonsils and lymph nodes (LNs), which are composed of leukocytes (95%) and architectural stromal cells (5%). LNs involved in mounting an immune response recruit large numbers of lymphocytes and support the division of those that recognise the foreign antigen, raising the question of how LN stromal cells manage this tremendous remodeling. In this study, we focused on specific zones within the lymph node called germinal centres that comprise dense aggregates or follicles of B lymphocytes, and investigated how lymphoid stromal cells contribute to the reorganization of primary B cell follicles into large reactive secondary follicles. Using a fate mapping system in mice, we identified a new stromal cell type that resides in the T cell zone of noninflamed resting LNs. We demonstrate that upon inflammation, B cells usually contained within B cell follicles progressively trespass into the adjacent T cell zone and surround and convert resident stromal cells into cells that can secrete CXCL13, a B cell chemokine. These CXCL13-secreting cells in turn act to delineate the new transient boundaries of the growing follicle. Identification of this distinct versatile stromal cell type adds to our understanding of mechanisms underlying compartmentalization of lymphoid organs into their functional zones.

Censoring of self-reactive B cells by follicular dendritic cell-displayed self-antigen

In the secondary lymphoid organs, intimate contact with follicular dendritic cells (FDCs) is required for B cell retention and Ag-driven selection during the germinal center response. However, selection of self-reactive B cells by Ag on FDCs has not been addressed. To this end, we generated a mouse model to conditionally express a membrane-bound self-antigen on FDCs and to monitor the fate of developing self-reactive B cells. In this article, we show that self-antigen displayed on FDCs mediates effective elimination of self-reactive B cells at the transitional stage. Notwithstanding, some self-reactive B cells persist beyond this checkpoint, showing evidence of Ag experience and intact proximal BCR signaling, but they are short-lived and unable to elicit T cell help. These results implicate FDCs as an important component of peripheral B cell tolerance that prevents the emergence of naive B cells capable of responding to sequestered self-antigens.

Primary antibody deficiencies

Primary antibody deficiencies (PADs) are the most common inherited immunodeficiencies in humans. The use of novel approaches, such as whole-exome sequencing and mouse genetic engineering, has helped to identify new genes that are involved in the pathogenesis of PADs and has enabled the characterization of the molecular pathways that are involved in B cell development and function. Here, we review the different PADs in terms of their known or putative mechanisms, which can be B cell intrinsic, B cell extrinsic or not defined so far. We also describe the clinical manifestations (including susceptibility to infections, autoimmunity and cancer) that have been associated with the various PADs.

Safe TNF-based antitumor therapy following p55TNFR reduction in intestinal epithelium

TNF has remarkable antitumor activities; however, therapeutic applications have not been possible because of the systemic and lethal proinflammatory effects induced by TNF. Both the antitumor and inflammatory effects of TNF are mediated by the TNF receptor p55 (p55TNFR) (encoded by the Tnfrsf1a gene). The antitumor effect stems from an induction of cell death in tumor endothelium, but the cell type that initiates the lethal inflammatory cascade has been unclear. Using conditional Tnfrsf1a knockout or reactivation mice, we found that the expression level of p55TNFR in intestinal epithelial cells (IECs) is a crucial determinant in TNF-induced lethal inflammation. Remarkably, tumor endothelium and IECs exhibited differential sensitivities to TNF when p55TNFR levels were reduced. Tumor-bearing Tnfrsf1a⁺⁺/⁻ or IEC-specific p55TNFR-deficient mice showed resistance to TNF-induced lethality, while the tumor endothelium remained fully responsive to TNF-induced apoptosis and tumors regressed. We demonstrate proof of principle for clinical application of this approach using neutralizing anti-human p55TNFR antibodies in human TNFRSF1A knockin mice. Our results uncover an important cellular basis of TNF toxicity and reveal that IEC-specific or systemic reduction of p55TNFR mitigates TNF toxicity without loss of antitumor efficacy.

Ablation of the cellular prion protein, PrPC, specifically on follicular dendritic cells has no effect on their maturation or function

Follicular dendritic cells (FDC) are situated in the primary follicles of lymphoid tissues where they maintain the structural integrity of the B-lymphocyte follicle, and help to drive immunoglobulin class-switch recombination, somatic hypermutation and affinity maturation during the germinal centre response. FDC can also provide a reservoir for pathogens that infect germinal centres including HIV and prions. FDC express high levels of the normal cellular form of the prion protein (PrP(C) ), which makes them susceptible to prion infection. The function of PrP(C) is uncertain and it is not known why FDC require such high levels of expression of a protein that is found mainly on cells of the central nervous system. In this study, the function of FDC was assessed in mice that had PrP(C) ablated specifically in their FDC. In mice with FDC-specific PrP(C) ablation, our analysis revealed no observable deficits in lymphoid follicle microarchitecture and FDC status. No effects on FDC ability to trap immune complexes or drive antigen-specific antibody responses and affinity maturation in B lymphocytes were observed. These data clearly demonstrate that PrP(C) expression is dispensable for the functional maturation of FDC and their ability to maintain antigen-specific antibody responses and affinity maturation.

Lymphotoxin α1β2 expression on B cells is required for follicular dendritic cell activation during the germinal center response

CD19-deficient mice were used as a model to study follicular dendritic cell (FDC) activation because these mice have normal numbers of FDC-containing primary follicles, but lack the ability to activate FDCs or form GCs. It was hypothesized that CD19 expression is necessary for B-cell activation and upregulation of membrane lymphotoxin (mLT) expression, which promotes FDC activation. Using VCAM-1 and FcγRII/III as FDC activation markers, it was determined that the adoptive transfer of CD19(+) wild-type B cells into CD19-deficient hosts rescued GC formation and FDC activation, demonstrating that CD19 expression on B cells is required for FDC activation. In contrast, CD19(+) donor B cells lacking mLT were unable to induce VCAM-1 expression on FDCs, furthermore FcγRII/III upregulation was impaired in FDCs stimulated with mLT-deficient B cells. VCAM-1 expression on FDCs, but not FcγRII/III, was rescued when CD19-deficient B cells expressing transgenic mLT were cotransferred into recipient mice with CD19(+) , mLT-deficient B cells, suggesting that FDC activation requires the CD19-dependent upregulation of mLT on activated B cells. Collectively, these data demonstrate that activated B cells are responsible for the initiation of FDC activation resulting in a microenvironment supportive of GC development and maintenance.

Fate determination of mature autoreactive B cells

A large antibody repertoire is generated in developing B cells in the bone marrow. Before these B cells achieve immunocompetence, those expressing autospecificities must be purged. To that end, B cells within the bone marrow and just following egress from the bone marrow are subject to tolerance induction. Once B cells achieve immunocompetence, the antibody repertoire can be further diversified by somatic hypermutation of immunoglobulin genes in B cells that have been activated by antigen and cognate T cell help and have undergone a germinal center (GC) response. This process also leads to the generation of autoreactive B cells which must be again purged to protect the host. Thus, B cells within the GC and just following egress from the GC are also subject to tolerance induction. Available data suggest that B cell intrinsic processes triggered by signaling through the B cell receptor activate tolerance mechanisms at both time points. Recent data suggest that GC and post-GC B cells are also subject to B cell extrinsic tolerance mechanisms mediated through soluble and membrane-bound factors derived from various T cell subsets.

Toll-like receptor 2-mediated intestinal injury and enteric tumor necrosis factor receptor I contribute to liver fibrosis in mice

BACKGROUND & AIMS

Progression of liver fibrosis in experimental models depends on gut-derived bacterial products, but little is known about mechanisms of disruption of the mucosal barrier or translocation. We used a mouse model of cholestatic liver disease to investigate mechanisms of intestinal barrier disruption following liver injury.

METHODS

Liver fibrosis and bacterial translocation were assessed in Toll-like receptor 2 (TLR2)-deficient and tumor necrosis factor receptor I (TNFRI)-deficient mice subjected to bile duct ligation. Epithelial and lamina propria cells were isolated and analyzed by immunoblot analyses and flow cytometry. We analyzed bone marrow chimeras and mice with a conditional gain-of-function allele for the TNFRI receptor. By crossing TNFRI(flxneo/flxneo) mice with mice that expressed the VillinCre transgene specifically in intestinal epithelial cells, we created mice that express functional TNFRI specifically on intestinal epithelial cells (VillinCreTNFRI(flxneo/flxneo) mice).

RESULTS

Following bile duct ligation, TLR2-deficient mice had less liver fibrosis and intestinal translocation of bacteria and bacterial products than wild-type mice. Mice with hematopoietic cells that did not express TLR2 also had reduced bacterial translocation, indicating that TLR2 expression by hematopoietic cells regulates intestinal barrier function. The number of TLR2(+) monocytes that produce tumor necrosis factor α increased in the intestinal lamina propria of wild-type mice following bile duct ligation; bacterial translocation was facilitated by TNFRI-mediated signals on intestinal epithelial cells.

CONCLUSIONS

Intestinal inflammation and bacterial translocation contribute to liver fibrosis via TLR2 signaling on monocytes in the lamina propria and TNFRI signaling on intestinal epithelial cells in mice. Therefore, enteric TNFRI is an important mediator of cholestatic liver fibrosis.